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Have you got a plan to deal with the moisture from that internal air flowing through timbers or breaks around partition walls, sockets, joints etc then condensing when it meets your wall which will be colder due to the IWI?
My worry with this setup would be that the PIR will simultaneously stop your walls from drying towards the interior and lower heat flow through them, reducing their ability to dry and upping their moisture content. Which will speed up the rotting process of any timber in the wall.
This coupled with the point @apc raises about the lack of an external finish to keep rain out of the wall does seem to present some risk factors for interstitial condensation to me based on what I know so far.
Not wanting to poo on your parade at all and hopefully this is helpful*. You've clearly done a lot of research and IIRC you're getting a MVHR system which might make this theoretical internal moisture exactly that?!
*In due course I'd quite like some constructive criticism on whatever system I decide on because this stuff is flippin' complicated 😵💫
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I dream of a cavity wall tbh! It would make our house a lot warmer and easier to insulate.
Is this purely because you can get cavity wall insulation? (With a cavity)
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If your setup works, this is what we'll be doing in space-year 2030 when we go all in on underfloor / heatpump / shitty extension overhaul.
Probably will have to internally insulate the crappy loft conversion though. The dot-dab plasterboard directly on the chimney stack acts as a lovely heatsink.
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The PIR will be adhered to the masonry with a foam seal around the perimeter of each board, along with a coat of liquid airtightness membrane. As you mentioned, continuous MVHR ventilation with auto-humidistat will be controlling humidity levels also.
The external brickwork will be properly re-pointed with lime mortar, and the walls in question are south-facing, so very unlikely to ever soak through (they haven’t in the 8 years I’ve been in the house).
andos
Howard
ectoplasmosis
I'm about to put in interior insulation on a few external walls in our solid-walled Victorian semi, and will likely use PIR instead of wood fibre.
Going by various calculators, the dew point will remain inside the PIR sheet itself, and applying the sheets properly, using foam all round, finishing with airtight tape, will/should prevent moist internal air from ever reaching the brick behind the boards. 50mm PIR will/should perform much better than 60mm wood fibre, and is significantly cheaper to boot.